LCL CAPACITOR CURRENT COMPENSATION AND CONTROL METHOD BASED ON DIVISION AND SUMMATION TECHNIQUE
An LCL capacitor current compensation and control method based on division and summation technique, comprising following steps: calculating new reference current i*lr=power grid reference current (Igr)+estimated capacitor current (); calculating duty cycle ratio d of respective switches in inverter to obtain inductor current (il), through using corresponding division-and-summation digital control characteristic equation (A), (B), (C), or (D), as based on inverter code of various inverter types; calculating power grid current (ig)=inductor current (il)−capacitor current (ic); calculating voltage across inductor at power grid side (vc−vp)=impedance (Zg) of said inductor at power grid side x power grid current (ig); utilizing equation (4) to calculate voltage across capacitor (vc); estimating capacitor current ()=voltage across said capacitor (vc)/filtering capacitor impedance (Zc); and utilizing equation (3) to estimate capacitor current ().
The present invention relates to an LCL capacitor current compensation and control method, and in particular to an LCL capacitor current compensation and control method based on division and summation technique.
THE PRIOR ARTSWith the advent of the Industrial Revolution since the 18th century, the petrochemical fuel, such as coal, petroleum, and natural gas are utilized and consumed in huge quantities. However, due to the exploitation for more than two hundred years, the energy resources are near depletion, and the energy crisis is getting serious. According to a survey conducted recently, with the petroleum resources presently available, it can only sustain industrial development and consumption for several decades before it comes to a complete depletion. In addition, for the uranium used for nuclear power generation, that will also be used up in the coming decades. Therefore, the green energy power generation is considered as an ideal and alternative energy resource, and thus it has caught much of the attention as the hope of energy development for the future.
In general, the electricity produced by green energy power generation (such as solar energy power generation, wind energy power generation . . . ) can be handled in two approaches: stored in a battery, or merged directly into a power grid of a power company. Wherein, the shortcoming of utilizing batteries is that, its power storage capacity and service life are limited, and its cost is high. Moreover, in case the power produced by green energy resources is merged directly into a power grid of a power company (such as Taiwan Power Company) through an inverter, the power loss during battery charging and discharging can be eliminated, to raise efficiency of power generation.
In merging the power produced by green energy resources into a power grid, the merged grid current of the inverter must conform to certain Specification, such as IEEE 1547.2-2008 and 519-1992. Therefore, there are stringent demands and restrictions on the harmonics of such a merged grid current In general, the harmonic distortion must be reduced to 3˜5% . In order to fulfill the requirement of this specification, an Inductor-Capacitor-Inductor (LCL) filter has to be added between the inverter and the power grid.
In this respect, refer to
In the documents of the prior art, a SPWM or SVPWM modulation approach is proposed, to perform analysis and control by considering the inverter a voltage source, such as Adaptive Current Control, PR Control, and Current Feedback Control. Though this type of control and strategy is capable of effectively improving the harmonic problem of the merged grid current, yet it has not dealt with the stability of LCL inverter output impedance and power grid impedance in a merged power grid. In another prior art, a stability criteria is proposed for inverter vs impedance. In a further prior art, an impedance shaping method is proposed to further deal with the problem of impedance stability, to raise the stability of merging an inverter into a power grid. Though this method proves to be feasible, yet the phase angle of its output impedance is very close to −90 degrees at low frequency, and this could lead to system oscillation, when the grid impedance is high. In addition, in the design of a controller, it fails to take into consideration that the inductance of the inverter can be varied with current, and this could lead to system instability in high power application.
Therefore, presently, the design and performance of merged power grid using LCL inverter is not quite satisfactory, and it has much room for improvement.
SUMMARY OF THE INVENTIONIn view of the problems and drawbacks of the prior art, the present invention provides an LCL capacitor current compensation and control method based on division and summation technique, to suppress or eliminate the harmonics in currents of a merged power grid.
In general, the ideal and standard value of power voltage provided by a Power Company, such as Taiwan Power Company is 220V of 60 Hz, with zero harmonics. Any deviation from this value is considered as power grid instability. In practice, the voltage of power provided by Taiwan Power Company is 220V+10%, having voltage frequency 60 Hz+1%, with 3% to 5% harmonics.
The source of harmonics could come from the power generation equipment during power generation, or it could also come from the device using the power in the power grid. In a power grid, both voltage and current could produce harmonics. Wherein, the voltage harmonics can be handled by using a voltage stabilization technique; while the present invention mainly deals with harmonics in a current, and in particular, it aims at suppressing or eliminating harmonics in a current of a merged power grid to avoid distortion, in achieving ideal sine waves.
In order to overcome the shortcomings and drawbacks of the prior art, the present invention provides an LCL capacitor current compensation and control method based on division and summation technique (FCCC). The compensation and control method takes into considerations of inductance variations, and it views an inverter as a current source. As such, through modifying reference current of inverter to compensate for the capacitor current for a distorted voltage, to suppress harmonics in current of a merged power grid, so as to achieve ideal sine wave of the current. The method of the present invention has the characteristics of accurately tracing power grid current, achieving high voltage harmonic suppression ratio, and realizing high stability tolerance.
In order to achieve the objective mentioned above, the present invention provides an LCL capacitor current compensation and control method based on division and summation technique, including the following steps: calculating the new reference current i*lr=power grid reference current (Igr)+estimated capacitor current () (step 1); calculating the duty cycle ratio d of the respective switches in the inverter to obtain the inductor current (il), through using the corresponding division-and-summation digital control characteristic equation (A), (B), (C), or (D), as based on the inverter code of various inverter types (step 2); calculating the power grid current (ig)=inductor current (il)−capacitor current (ic) (step 3); calculating voltage across inductor at power grid side (vc−vp)=impedance (Zg) of inductor at power grid side x power grid current (ig) (step 4); utilizing equation (4) to calculate voltage across capacitor () (step 5); estimating capacitor current ()=voltage across capacitor (vc)/filtering capacitor impedance (Zc) (step 6); and utilizing equation (3) to estimate capacitor current () (step 7).
In the present invention, the types of inverter utilized in implementing the LCL capacitor current compensation and control method based on division and summation technique may include the following: single-phase double-wire bi-directional inverter (
three-phase three-wire bi-directional inverter (
In step 2 mentioned above, in case the inverter code is A01, then execute the division-and-summation digital control characteristic equation (A); in case the inverter code is A02, then execute the division-and-summation digital control characteristic equation (B); in case the inverter code is A03, then execute the division-and-summation digital control characteristic equation (C); and in case the inverter code is A04, then execute the division-and-summation digital control characteristic equation (D), in achieving the objective of suppressing harmonics in a merged grid current.
The advantages of the present invention are that, the compensation and control method takes into considerations of inductance variations, and it views an inverter as a current source. As such, through modifying reference current of inverter to compensate for the capacitor current for a distorted voltage, it is able to suppress harmonics in current of a merged power grid, so as to achieve ideal sine wave of the current. The method of the present invention is characterized in that, it is capable of accurately tracing power grid current, achieving high voltage harmonic suppression ratio, and realizing high stability tolerance. As such, it could avoid the drawbacks and shortcomings of the prior art that, the harmonics in a power grid current leading to degradation of sensitivity and accuracy for the various electronic devices receiving and using the current in the merged power grid.
Further scope of the applicability of the present invention will become apparent from the detailed descriptions given hereinafter. However, it should be understood that the detailed descriptions and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the present invention will become apparent to those skilled in the art from this detailed descriptions.
The related drawings in connection with the detailed descriptions of the present invention to be made later are described briefly as follows, in which:
The purpose, construction, features, functions and advantages of the present invention can be appreciated and understood more thoroughly through the following detailed description with reference to the attached drawings.
It has to be noted that, in the present invention, numerous equations are utilized to explain the contents of the present invention, and two of them are quite voluminous as to each occupies a whole page. For this reason, all of those equations are listed at the end of the Specification to facilitate the reader to conduct continuous reading without being distracted by the equations.
In the descriptions of embodiments 1 to 4, the following systems are used: single-phase double-wire bi-directional inverter system, three-phase four-wire bi-directional inverter system, three-phase three-wire bi-directional inverter system, and single-phase three-wire bi-directional inverter system, and that all belong to the prior art. In using the systems mentioned above to merge the self-generated electric power into a local power grid, the current of the merged power grid tends to produce harmonics, and it is thus seriously distorted. However, when utilizing the systems mentioned above to implement the LCL capacitor current compensation and control method based on division and summation technique of the present invention, the harmonics in the current of the merged power grid can be suppressed or eliminated, to produce ideal sine waves of current. As such, it can avoid the drawbacks and shortcomings of the prior art in that, in the prior art, the electronic devices receiving and using such merged power grid current will be adversely affected by the harmonics, such that its sensitivity and accuracy tend to degrade. Through using the compensation and control method of the present invention, the drawbacks and shortcomings of the prior art can be avoided.
First Embodiment Single-Phase Double-Wire Bi-Directional Inverter SystemFirstly, refer to
As shown in
In addition, one end of voltage feedback circuit 130 is connected to the LCL filter 120 and the single-phase double-wire bi-directional inverter 110, to receive from them the feedback voltage vdc, vc, and vp, and transmit the feedback voltage to a single chip micro-controller 160, connected to the other end of the voltage feedback circuit 130. Moreover, current feedback circuit 150 is connected to LCL filter 120, to receive from it the feedback current il, and transmit the feedback current to a single chip micro-controller 160, connected to the other end of the current feedback circuit 150. Further, one end of driving circuit 140 is connected to the single chip micro-controller 160, to receive from it instructions, and transmit the control signals SA+, SA−, SB+, and SB− to single-phase double-wire bi-directional inverter 110, connected to the other end of the driving circuit 140.
In the descriptions above, in the circuit and control block diagram for a single-phase double-wire bi-directional inverter system, the current tracing instruction (Igr) of the inverter current (il) is of a sine wave function at basic frequency (refer to equation 1). Through using the division-and-summation control, the inverter may accurately follow the current tracing instructions (Igr). However, when the power grid connected to the inverter does have marked voltage harmonics, the power grid current (ig) will have serious distortions. When the grid voltage contains harmonics, the capacitor current (ic) in the filter will also have harmonics. In order to merge the sine wave current of basic frequency into the power grid, the current tracing instruction (Igr) of the inverter current (il) must be updated to a new reference current (i*lr), as shown in Equation 1. Wherein, Igr is a current tracing instruction and it is also a power grid reference current, as shown in Equation 2, such that is an estimated capacitor current, and it can be obtained through using Equation 3. Equation 4 is used to estimate the vc(n+1) in Equation 3, wherein, vc(n) is the sensed capacitor voltage, vp(n) is the sensed duty division point (PCC) voltage; Igr and vp are of the same phase, and IM is the amplitude of Igr.
Then, refer to
As shown in
1. merged power grid reference current; 2. estimated capacitor current required to be compensated; 3. inductor reference current traced by the inverter; 4. real feedback value of inductor current; 5. deviation between inductor current needs to be traced and the real value; 6. compensation gain of the controller; 7. variations of duty cycle ratio; 8. amplification gain of the circuit; 9. variations of inductor current of a circuit; 10,14 delayed a switching cycle TS in digital control, namely, the value of previous cycle can be used in the present cycle; 11. impedance Zc=1/sCs in filter capacitance frequency domain; 12. impedance Zg=sLg at the power grid side inductor frequency domain; 13
in
as realized in digital control; 15. inductor current feedback ratio, usually is 1; 16,17,18,19,20,23. adder; 21. duty division point voltage; 22. real merged power grid current.
Subsequently, refer to
In the present invention, the types of inverter utilized in implementing the LCL capacitor current compensation and control method based on division and summation technique of the present invention may include the following: single-phase double-wire bi-directional inverter (
three-phase three-wire bi-directional inverter (
In step 320 mentioned above, in case the inverter code is A01, then execute the division-and-summation digital control characteristic equation (A); in case the inverter code is A02, then execute the division-and-summation digital control characteristic equation (B); in case the inverter code is A03, then execute the division-and-summation digital control characteristic equation (C); and in case the inverter code is A04, then execute the division-and-summation digital control characteristic equation (D).
It is worth to note that, in implementing the LCL capacitor current compensation and control method, since in the present embodiment, a single-phase double-wire bi-directional inverter is used, with its inverter code as A01. Therefore, it utilizes the corresponding division-and-summation digital control characteristic equation (A) to calculate inductor current (il) and estimate capacitor current (), and then make compensation (il−) for the output current of the inverter, as such suppressing or eliminating harmonics, and producing grid current (ig) of ideal sine wave.
In the descriptions above,
Then, refer to
Refer to
Further, in table 1, PF means Power Factor; VTHD means voltage Total Harmonic Distortion; and ITHD means Current Total Harmonic Distortion.
In the following, refer to
Refer to
As shown in
The structure and configuration of the present embodiment are similar to that of the single-phase double-wire bi-directional inverter system 100 of the first embodiment as shown in
Wherein, the first switch is connected to the second switch, with their connection point connected to the first set inductor-capacitor-inductor (LCL) at the positive end of LCL filter. the third switch is connected to the fourth switch, with their connection point connected to the second set (LCL) at the positive end of LCL filter. The fifth switch is connected to the sixth switch, with their connection point connected to the third set inductor-capacitor-inductor (LCL) at the positive end of LCL filter. The seventh switch is connected to the eighth switch, with their connection point connected to the fourth set inductor-capacitor-inductor (LCL) at the positive end of LCL filter.
In addition, the direct current chain voltage feedback circuit 730 is connected between the three-phase four-wire bi-directional inverter 710 and the single chip micro-controller 770, for receiving feedback voltage VDC, and transmitting it to the single chip micro-controller 770. The driving circuit 740 is connected between the three-phase four-wire bi-directional inverter 710 and the single chip micro-controller 770, for transmitting the driving signals M1 and MR to the three-phase four-wire bi-directional inverter 710. The current feedback circuit 750 is connected between the three-phase four-wire bi-directional inverter 710 and the single chip micro-controller 770, for receiving feedback current ILM. The voltage feedback circuit 760 is connected between the LCL filter 720 and the single chip micro-controller 770, for receiving feedback voltage vp and vc.
The operation principle of the three-phase four-wire bi-directional inverter 710 and LCL filter 720 of the present embodiment is similar to that of the single-phase double-wire bi-directional inverter system 100 and the LCL filter 120 of the first embodiment, people familiar with this field can infer to know it easily, so it will not be repeated here for brevity. Yet, it worth to note that, in the present embodiment, in performing the LCL capacitor current compensation and control method of the present invention, namely, in performing the method of
Furthermore, the
Refer to
As shown in
The structure and configuration of the present embodiment are similar to that of the single-phase double-wire bi-directional inverter system 100 of the first embodiment as shown in
Wherein, the first switch is connected to the second switch, with their connection point connected to the first set inductor-capacitor-inductors (LCL) at the positive end of LCL filter. the third switch is connected to the fourth switch, with their connection point connected to the second set (LCL) at the positive end of LCL filter. The fifth switch is connected to the sixth switch, with their connection point connected to the third set inductor-capacitor-inductors (LCL) at the positive end of LCL filter.
In addition, the direct current chain voltage feedback circuit 830 is connected between the three-phase three-wire bi-directional inverter 810 and the single chip micro-controller 870, for receiving feedback voltage VDC, and transmitting it to the single chip micro-controller 870. The driving circuit 840 is connected between the three-phase three-wire bi-directional inverter 810 and the single chip micro-controller 870, for transmitting the driving signals M1 and MR to the three-phase three-wire bi-directional inverter 810. The current feedback circuit 850 is connected between the three-phase three-wire bi-directional inverter 810 and the single chip micro-controller 870, for receiving feedback current ILM, and transmitting it to the single chip micro-controller 870. The voltage feedback circuit 860 is connected between the LCL filter 820 and the single chip micro-controller 870, for receiving feedback voltage vp, and vc.
The operation principle of the three-phase three-wire bi-directional inverter 810 and LCL filter 820 of the present embodiment is similar to that of the single-phase double-wire bi-directional inverter system 100 and the LCL filter 120 of the first embodiment, people familiar with this field can infer to know it easily, so it will not be repeated here for brevity. Yet, it worth to note that, in the present embodiment, in performing the LCL capacitor current compensation and control method of the present invention, namely, in performing the method of
Furthermore, the
Refer to
As shown in
The structure and configuration of the present embodiment are similar to that of the single-phase double-wire bi-directional inverter system 100 of the first embodiment as shown in
Wherein, the first switch is connected to the second switch, with their connection point connected to the first set of inductor-capacitor-inductors (LCL) at the positive end of LCL filter. the third switch is connected to the fourth switch, with their connection point connected to the second set (LCL) at the positive end of LCL filter.
In addition, one end of the voltage feedback circuit 930 is connected to the single-phase three-wire bi-directional inverter 910 and the LCL filter 920, to receive from them the feedback voltage Vdc, VCA, VCB, vpA, vpB, and transmit the voltages to the single chip micro-controller 960 connected to its other end. Further, current feedback circuit 950 is connected to the LCL filter 920, to receive from it the feed back current i1A and i1B, and transmit them to the single chip micro-controller 960. Moreover, one end of the driving circuit 940 is connected to the single chip micro- controller 960, to receive from it instructions, and then transmit the control signals SA+, SA−, SB+, and SB− to the single-phase three-wire bi-directional inverter 910 connected to the other end of the driving circuit 940.
The operation principle of the single-phase three-wire bi-directional inverter 910 and LCL filter 920 of the present embodiment is similar to that of the single-phase double-wire bi-directional inverter system 100 and the LCL filter 120 of the first embodiment, people familiar with this field can infer to know it easily, so it will not be repeated here for brevity. Yet, it worth to note that, in the present embodiment, in performing the LCL capacitor current compensation and control method of the present invention, namely, in performing the method of
Furthermore, the
In the first embodiment to the fourth embodiment mentioned above, the essence of design of the micro-controller is to integrate a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), an input/output device (I/O), and an analog-to-digital converter (A/D) on a single chip, such that it has the function of a micro computer. In the embodiments mentioned above, the steps and equations utilized in the LCL capacitor current compensation and control method based on division and summation technique, can be stored in the memory of the single chip micro-controller as shown in
Summing up the above, the present invention provides an LCL capacitor current compensation and control method based on division and summation technique, to overcome the shortcomings and drawbacks of the prior art. The compensation and control method takes into considerations of inductance variations, and it views an inverter as a current source. As such, through modifying reference current of inverter to compensate for the distorted capacitor current caused by a distorted voltage, it is able to suppress harmonics in current of a merged power grid, so as to achieve ideal sine wave. The method of the present invention is capable of avoiding the deficiency of the prior art that, degradation of the sensitivity and accuracy of the electronic device receiving and using this type of merged grid current as caused by the harmonics contained therein. Further, the method of the present invention is able to achieve the objective of accurately tracing power grid current, attaining high voltage harmonic suppression ratio, and realizing high stability tolerance. Therefore, the present invention is able to attain the effect not anticipated in the prior art, thus fulfill patent requirements and has patent value.
The above detailed description of the preferred embodiment is intended to describe more clearly the characteristics and spirit of the present invention. However, the preferred embodiments disclosed above are not intended to be any restrictions to the scope of the present invention. Conversely, its purpose is to include the various changes and equivalent arrangements which are within the scope of the appended claims.
division-and-summation digital control characteristic equation (A):
division-and-summation digital control characteristic equation (B):
interval 0°˜60°:
interval 60°˜120°:
interval 120°˜180°:
interval 180°˜240°:
interval 240°˜300°:
interval 300°˜360°:
division-and-summation digital control characteristic equation (C):
interval 0°˜60°:
interval 60°˜120°:
interval 120°˜180°:
interval 180°˜240°
interval 240°˜300°
interval 300°˜360°
division-and-summation digital control characteristic equation (D):
Claims
1. An LCL capacitor current compensation and control method based on division and summation technique, comprising following steps:
- calculating a new reference current i*1r=power grid reference current (Igr)+estimated capacitor current ();
- calculating a duty cycle ratio d of respective switches in an inverter to obtain inductor current (i1), through using a corresponding division-and-summation digital control characteristic equation (A), (B), (C), or (D), as based on an inverter code of various inverter types;
- calculating a power grid current (ig)=inductor current (i1)−capacitor current (ic);
- calculating voltage across an inductor at a power grid side (vc−vp)=impedance (Zg) of said inductor at power grid side x power grid current (ig);
- utilizing equation (4) to calculate voltage across a capacitor (vc);
- estimating capacitor current ()=voltage across said capacitor (vc)/filtering capacitor impedance (Zc); and
- utilizing equation (3) to estimate a capacitor current ().
2. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 1, wherein said inverter includes following types: single-phase double-wire bi-directional inverter system, with its inverter code set at A01; three-phase four-wire bi-directional inverter system, with its inverter code set at A02; three-phase three-wire bi-directional inverter system, with its inverter code set at A03; and single-phase three-wire bi-directional inverter system, with its inverter code set at A04.
3. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 2, wherein in case said inverter code is A01, then said single-phase double-wire bi-directional inverter system executes said division-and-summation digital control characteristic equation (A).
4. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 2, wherein in case said inverter code is A02, then said three-phase four-wire bi-directional inverter system executes said division-and-summation digital control characteristic equation (B).
5. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 2, wherein in case said inverter code is A03, then said three-phase three-wire bi-directional inverter system executes said division-and-summation digital control characteristic equation (C).
6. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 2, wherein in case said inverter code is A04, then said single-phase three-wire bi-directional inverter system executes said division-and-summation digital control characteristic equation (D).
7. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 1, wherein said equation 4 is ( n + 1 ) = v p ( n ) + L g ( I g r ( n + 1 ) - I g r ( n - 1 ) ) T S
8. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 1, wherein said equation 3 is = C s ( v c ( n + 1 ) - v c ( n ) ) T S
9. The LCL capacitor current compensation and control method based on division and summation technique as claimed in claim 1, wherein said LCL capacitor current compensation and control method is used in said single-phase double-wire bi-directional inverter system, said three-phase four-wire bi-directional inverter system, said three-phase three-wire bi-directional inverter system, and said single-phase three-wire bi-directional inverter system, and through modifying reference current of an inverter to compensate for a capacitor current for a distorted voltage, to suppress harmonics in current of a merged power grid, so as to achieve ideal sine wave, to avoid deficiency of prior art that, degradation of the sensitivity and accuracy of an electronic device receiving and using this type of merged grid current as caused by the harmonics contained therein, as such achieving objective of accurately tracing power grid current, attaining high voltage harmonic suppression ratio, and realizing high stability tolerance.
Type: Application
Filed: Nov 13, 2014
Publication Date: Mar 3, 2016
Inventors: TSAI-FU WU (Hsin-chu City), LI-CHUN LIN (Hsin-Chu City), NING YAO (Hsin-Chu City)
Application Number: 14/540,039